1. Field of the Invention
The present invention relates to rivets for use in aerospace structures, and more particularly to rivets formed from friction-actuated extrusions of comminuted rapidly solidified aluminum base alloy powder.
2. Description of the Prior Art
In a "friction-actuated" extrusion process, metal is fed into one end of a passageway formed between first and second members, with the second member having a greater surface area for engaging the metal than the first member. The passageway has an obstruction at the end remote from the end into which the metal is fed. At least one die orifice of the passageway is associated with the obstructed end. The passageway-defining surface of the second member moves relative to the passageway-defining surface of the first member in the direction towards the die orifice from the first end to the obstructed end. Frictional drag of the passageway-defining surface of the second member draws the metal through the passageway and generates therewithin a pressure that is sufficient to extrude the metal through the die orifice. The obstructed end of the passageway may be blocked substantially entirely as described in British Patent Specification No. 1370894. In conventional practice, such as the conform process described in U.S. Pat. Nos. 4,552,520 and 4,566,303 the passageway is arcuate and the second member is a wheel with a grove formed in its surface. The first member projects into the grove and the obstructed end is defined by an abutment projecting from the first member. Preferably, the abutment member is of substantially smaller cross-section than the passageway, so that it leaves a substantial gap between the abutment member and the groove surface. In this case metal adheres to the groove surface, as described in UK Patent No. 2069389B, whereby a portion of the metal extrudes through the clearance and remains as a lining in the groove to re-enter the passageway at the entry end, while the remainder of the metal extrudes through the die orifice.
The conform process was originally developed for the extrusion of metal rod in-feed. Attempts have been made to provide an in-feed in the form of granules. The ability to extrude aluminum and/or aluminum alloys from granular in-feed has proven to be difficult because the aluminum powder does not have adequate flow to sustain the process. This is especially true for high performance aluminum alloys such as those prepared from inert or flue gas atomization or mechanical alloying. Alloy granules produced by these processes have morphologies that render the in-feed non-flowable. In addition, the high hardness of the granules makes the actual friction-actuated extrusion difficult. To avoid flow problems associated with aluminum alloy granules having high hardness, efforts have been made to conform in-feed composed of softer aluminum and/or aluminum alloy granules. In such processes, the soft aluminum granules quickly gum the apparatus and the extruded material is prone to blistering on the surface and failing at the particle surface (i.e., interparticle separation) due to the presence of an oxide layer in the granules. A process for providing a friction actuated extrusion using rapidly solidified and comminuted aluminum alloy as the in-feed to the extruder has been disclosed in U.S. Pat. No. 4,898,612.
At present the riveting of aluminum aircraft structures that are heated either by aerodynamic heating or are in close proximity to the aircraft engines requires the work of stainless steal, nickel base alloy or titanium alloy fasteners. However the material compatibility of the interface fastener with the aluminum structure is a concern for several reasons. Thermal expansion over the wide range of intended operating temperatures is significant. The reliable interface values are best maintained if the rivet and the structural material each have the same coefficient of thermal expansion. If the yield strength of the structural sheet is much less than that of the rivet shank, the surface rivet will often be dimpled during installation. Dimpling is particularly troublesome with thin stack ups. There is a significant weight penalty in using heavier rivets. Rework is very difficult in structures assembled with upset as fasteners having higher strength than the part itself. Drilling a hard fastener out of a softer plate often results in irregular holes in the plate.
Fasteners formed from ingot cast aluminum alloys cannot be used because at temperatures above 150.degree. C. they lose a significant fraction of their strength or are so hard that they cannot be cold headed.